Tectonic Evolution And Its Influence On The Ordovician Oil And Gas Accumulation Conditions In Southwest Ordos Basin

The southwest Ordos Basin margin located in the transition zone between the Ordos craton and active Qilian and Qinling orogenic belt, has experienced some stages and difference of tectonic deformation and uplift-denudation reformation through the periods of Caledonian, Yanshan and Ximalay, the tectonic transformation relations, the differences of tectonic evolution-transform and their effect on the constraints of the rebuilding and conservation state in the southern and western margin of the basin, especially the geological conditions of the mainly marine petroleum strata in the Ordovician, these are hot topics about structural dynamics research and marine oil and gas exploration in the Ordos Basin in recent years. The Qinglongshan-Pingliang thrust belt in the southern section of west margin of basin and the uplift belt of Weihe river in the southern margin of the basin as the key research zone of this paper, we combined the methods of the field geological survey of structural section, seismic profile interpretation and tectonic thermal chronology and analysed comprehensive tectonic evolution-transform stage, structural deformation and their effect on hydrocarbon source-reservoir factors and reservoir conditions in the Ordovician strata after the deposition time of the Ordovician marine sediments of the Early Paleozoic in the southwestern margin of the Ordos Basin.Through the field geological structure section and comprehensive analysis about seismic-geophysical profile, we has been clear about the main fault structure characteristics, the fracture tectonic system and their relationship with adjacent active orogenic belt in the southwest edge of the Ordos Basin. We though the Qinglongshan-Pingliang fracture the Shenrenqiao-Tongguan fracture in the central and south of the Weibei bulge are boundary faults of the thrust-fold structure composite basin-mountain in the late Caledonian and the medium Yanshan period; the Lingtai-Yijun-Huangling fault in the north margin of the Weibei bulge is characterized by high angle thrust in the middle Yanshan period, which is the tectonic boundary faults between the Weibei bulge in the southern margin of the basin and the stable sedimentation unit in the basin.We clarified the main episodes of tectonic deformation and tectonic deformation characteristics in the southwest edge of the Ordos Basin, through interpreting the geological structure and screening the time of tectonic deformation of the remaining structural feature in the southwest margin of the basin, especially the southern margin of the basin and combining the regional stratigraphic unconformity relationships with the analysis of the deformation characteristics of different tectonic epochs. We at least determined three major tectonic deformation events in the study area, including the Caledonian brittle-ductile overthrust structure, the middle Yanshan brittle thrust-fold tectonic deformation and the Himalayan mid-late slip thrust structures (in the western margin of the basin) and different break long (in the southern margin of the basin)We analysed comprehensively the Evolutionary history profile and the top surface of the Ordovician tectonic evolution and determined and concluded that the late Caledonian tectonic deformation effect caused the marine sedimentary wedge of Ordovician (located in the southwestern of the Ordos Plot (Basin) and belonged to the platform foreslope facies) to thrust and uplift (the prototype of Weibei uplift), which became a wider range of the top surface of the Ordovician uplift and a ramp volts toward north combining with the Ordovician platform edge uplift (the central paleouplift), and controled and affected the ancient karst landforms from the late Ordovician to early Carboniferous and the sediment-tectonic evolution of the basin since late Paleozoic in the study area.The tectonic thermal chronology anysis, apatite fission track thermal history simulation and vitrinite reflectance (Ro) paleogeothermal gradient recovery results showed that the supreme paleogeothermal field period mainly occurred in middle Yanshan period (140～110Ma±) in the southwest Ordos Basin margin, the paleogeothermal gradient is close to 40 ℃/km, which is the master tectonic thermal events of the large mature hydrocarbon expulsion from the Ordovician hydrocarbon source rocks in the study area; the late evolution-transformation process mainly experienced the late Yanshan (95-85Ma and 70-65Ma) and the Himalayan Epoch (60-40Ma and 30-25Ma), especially significant tectonic uplift cooling events of 10-5Ma since the late Cretaceous.Multiphase tectonic deformation events, the different intensity zoning of the late uplift-erosion and the comprehensive analysis of the rebuilding and preservation condition of the Ordovician oil and gas geological conditions, we concluded that the structure relatively stable western-leaning gentle slope zone in the north of Weibei uplift is the relatively favorable zones, which include more development of the Ordovician dolomite-ancient karst reservoir and is the destination of the oil and gas migration and accumulation from the Palaeozoic hydrocarbon source rocks; the Ordovician reef-beach facies carbonate reservoir in the Linyou-Binxian and the Ordovician top surface medium and small broken nose or faulted anticline structure in the weak transformation gentle slope area of the north wing of the Weibei uplift is probably the favorable zones of the Ordovician lithology or lithologic-structural oil and gas trap in the southern margin of the basin; In the Structure conversion section in the western margin of the southern and southern margin of the western of basin, the structure thrust and denudation transformation is relatively strong in the Ordovician Marine strata tectonic and oil and gas geological conditions are relatively poor.